The alkali metal salts of 2-pyrrolidone-5-carboxylic acid (pyroglutamic acid) are of considerable interest. For example, sodium pyroglutamate in particular is used widely as an agent for moisture retention. It is extremely hygroscopic and can absorb over 60% of its own weight of water within 30 days in air which has 65% relative humidity. A few years ago, L-sodium pyroglutamate was found to be a component of the "natural moisturizing factor" of human skin [K. Laden, R. Spitzer, "J. Soc. Cosmetic Chemists", 18, p. 351, (1967); K. Laden, "American Perfumer and Cosmetics", 82, p. 77, 1967].
Therefore, it is not surprising that sodium pyroglutamate is currently a component of numerous cosmetic products such as skin creams, hair tonics, perfumes, soaps, shampoos and toothpastes (see e.g. Japanese Patent Application JP-OS 50/25741; JP-OS 51/96808; JP-OS 55/49306 or JPOS 59/189197). Sodium pyroglutamate is also used for the moisture-retention of medical articles such as plasters and suppositories, in tobacco, in water-soluble inks or as intermediary product in chemical synthesis (see e.g. Published German Patent Application DE-OS 30 23 417).
However, in the past the synthesis of L-sodium pyroglutamate was relatively complicated. Thus, the cyclization of a mixture of L-glutamic acid and L-sodium glutamate in aqueous solution at 200.degree. C. and elevated pressure has been described in Published Japanese Patent Application JP-OS 51/110559. A disadvantage of this method is the fact that the sodium pyroglutamate is obtained as only a 50% solution in water and that a complete racemization to D,L product, occurs. The latter is undesirable because the natural moisturizing factor consists solely of L-sodium pyroglutamate.
Another known possibility is the reaction of pyroglutamic acid with sodium hydroxide in water (see Published German Patent Application DE-OS 21 63 939); however, only an aqueous solution of the salt is produced in this instance also. Moreover, this method nevertheless requires use of L-pyroglutamic acid. Pyroglutamic acid is formed by heating L-glutamic acid in concentrated aqueous solution, but the product is racemized and the reaction results in an equilibrium of starting material and final product. A separation step is required for the synthesis of pure L-pyroglutamic acid (see P. M. Hardy, "Synthesis", 1978, p. 290). The direct heating of L-glutamic acid in the melt to 180.degree. to 185.degree. C. is also accompanied by racemization and, especially, by very strong decomposition (see N. Lichtenstein, S. Gertner, "J. Am. Chem. Soc." vol. 64, pages 1021-1022, 1942.